Virtually all of the used nuclear fuel produced in the United States and a significant quantity generated overseas is stored in sealed canisters that have their parts welded together. These canisters are sometimes referred to as multi-purpose canisters (MPCs). MPCs are mostly manufactured from austenitic stainless steel but exotic alloys such as Hastaloy, Inconel, etc., have also been considered. The typical MPC includes two major parts, the first part being formed from an outer a cylindrical vessel having a shop-welded bottom (base plate) and a flat top lid which is welded to the top of the cylindrical vessel at the nuclear plant after the fuel is loaded inside the MPC. The outer body of the completed MPC is also referred to as an “enclosure vessel”. The second part is the internal structure, called a fuel basket, which stores the nuclear fuel in the desired configuration. An MPC may also be used to store other forms of high level waste, although for ease of discussion the term “spent fuel” is used to represent all forms of high level waste.
The enclosure vessel is responsible for maintaining confinement of its radiological contents including gaseous matter, under all normal, off-normal, and accident design conditions. The physical integrity of the pressure retention and confinement boundary of an MPC is a fundamental safety requirement during storage and transport. Accordingly, to ensure maximum protection against leakage, the enclosure vessel is made using the rules of ASME Section III Class 1, which is the most rigorous pressure vessel code in use in the United States. Pursuant to the provisions for Class 1 components in Section III of the Code, all pressure boundary material in the Enclosure Vessel is ultrasonically examined and all body welds are subject to 100% volumetric examination (e.g., radiography or ultrasonic testing (UT)). The state of the art, however, does not enable 100% volumetric examination of the top lid of the MPC because the top lid can only be welded in the field, after the canister has been loaded with spent fuel. The process of sealing the canister is therefore necessarily performed in the presence of a high radiation field around the canister so that weld crews have limited physical access to the canister when it is sealed due to the high radiation doses to which they would be exposed. Prompted by the need to prevent large dose exposure to the crew, the lid-to-shell (LTS) weld joint has historically been made as a partial penetration half-V groove or J-groove weld (see FIG. 1). Such a weld is readily made by an automated weld machine having a weld arm designed to traverse the circular weld-path. The drawback is that the partial penetration weld cannot be 100% volumetrically examined with an acceptable level of accuracy. However, the USNRC permits the root weld and successive passes to be examined using by the less robust method of liquid penetrant (LP) examination. Because LP is a surface examination tool, the soundness of the weld mass located between successive LP examinations cannot actually be examined.
Efforts to devise a UT process to volumetrically examine the partial penetration closure weld thus far have not been successful. State-of the-art UT technology is capable of providing high quality volumetric examination of the entire closure weld mass with only a small “blind spot” located at the tip of the root pass area. There are proposals to perform liquid penetrant examination of the root pass to confirm its quality, and after the root pass has been checked for soundness, the balance of the weld mass may be examined by UT. Regardless, although the risk of a hidden flaw in the closure weld propagating under a storage or transport event is extremely small, given the use of highly fracture resistant austenitic stainless steel material in the MPCs, the lack of the ability to subject the entire weld mass to 100% volumetric examination remains a weakness for enclosure vessels.